A bistable semiconductor laser is described in detail on a paper of "The Institute of Electronics, Information and Communication Engineers, 1985". In the bistable semiconductor laser, light is supplied from a light emitting diode through a lens to one facet thereof to be focussed into an active layer in an unexcited state so that a laser oscillation is obtained with the trigger light.
Such a bistable semiconductor laser in which a laser oscillation is performed with a minute trigger light, and wherein the laser oscillation thus induced is continued thereafter even in the absence of the trigger light, is applicable to an optical semiconductor memory which is an important device in a prospective optical switching system, parallel optical information processing system and so on.
Conventional optical semiconductor memory is described on pages 596 to 600 of "Journal of the Applied Physics, 59(2), 15 January 1986". The optical semiconductor memory includes a pnpn thyristor to be provided with an n-base layer of n-GaAs having a narrower forbidden bandwidth between an anode region of p-A1GaAs and cathode region of n-A1GaAs. In the thyristor, a bias voltage is applied thereto to provide a turning-on thereof wherein a transition is performed from a high impedance state to a low impedance state so that carriers are injected into the n-base region to be confined therein. As a result, a spontaneous emission light is radiated from the thyristor.
In the case of the bistable semiconductor laser, however, there is a disadvantage that an optical coupling of the trigger light is difficult at the facet thereof, thereby increasing the coupling loss of the trigger light, although an induced emission light which is desirable in regard to light output power is obtained therein.
In the case of the pnpn thyristor, further, such a high output of light is not obtained as in the bistable semiconductor laser because an output light is a spontaneous emission light.